Pressure Control at Variable Speed

In a specific application, a large, mixed hydrocarbon gas system may be set up with a turbine-driven compressor as shown in Figure 8-40(a).

The process demands that a constant pressure be held on the header at various loads (flow). Specifically, the control system operates as follows;

1. The pressure transmitter (transducer block) senses the process pressure. It converts this signal to a signal proportional to the process pressure and sends it to the pressure controller.

2. The pressure controller (controller block) amplifies the transmitter signal and sends a modified signal to the final element. Depending on the system requirements, the controller block may include additional correction factors, integral and derivative (reset and rate). This is called a three-mode controller.

3. The final element in this case is speed control. This mechanism varies the turbine-governor speed setting over a predetermined range.

A pressure rise in the reactor would cause an increase in process pressure above the set value, send a signal to the governor, and thus, reduce the speed, lower the flow, and thereby, maintain the desired system pressure.

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